1. Field of the Invention
The present invention relates to a steam turbine power generation system, which is provided with a steam turbine having a temperature of a driving steam raised to a high temperature, and a low-pressure turbine rotor.
2. Description of the Related Art
It is general that a steam turbine power generation system is provided with a high-pressure turbine, an intermediate-pressure turbine and a low-pressure turbine. A high-temperature, high-pressure driving steam supplied from a boiler flows into the high-pressure turbine, rotates the high-pressure turbine in high-pressure blade stages to perform expansion work and then is discharged out of the high-pressure turbine. The driving steam discharged from the high-pressure turbine is supplied sequentially to the intermediate-pressure turbine and the low-pressure turbine to rotate the individual turbines to perform expansion work, and discharged to a condenser for condensation to water.
In recent years, steam turbine power generation systems having a higher inlet steam temperature of the high-pressure turbine in order to improve a thermal efficiency are increasing, and they have a tendency that driving steam has a large difference in temperature between the inlet and the outlet of the steam turbine. To deal with the temperature difference, there are disclosed conventional steam turbine power generation systems which are provided with a steam turbine having, for example, a high-temperature material as a rotor material (e.g., Japanese Patent Laid-Open Applications No. Hei 09-287402, No. Hei 09-195701, No. 2003-27192 and No. 2004-36469) and a steam turbine having a cooling structure for the steam inlet portion of the steam turbine (e.g., Japanese Patent Laid-Open Applications No. 2000-328904 and No. 2004-36527).
As described above, the conventional steam turbine power generation systems have the steam temperature at the low-pressure turbine inlet set to a temperature at which mechanical strength of, for example, a material for the low-pressure turbine rotor can be maintained. It is mainly because considerable embrittlement due to aging or sometimes simultaneous embrittlement and softening are caused if the material for the conventional low-pressure turbine rotor has a temperature exceeding the temperature at which the mechanical strength can be maintained.
Therefore, where the driving steam temperature at the steam turbine inlet was raised to a high level, it was necessary to increase the expansion work load by the high-pressure turbine and the intermediate-pressure turbine to lower the driving steam temperature at the low-pressure turbine inlet to a temperature at which embrittlement of the low-pressure turbine rotor material due to aging or softening due to aging could be suppressed.
As a result, there was a disadvantage that the number of blade stages of the high-pressure turbine and the intermediate-pressure turbine was increased, resulting in increasing the whole turbine size. And, the increase in number of stages of the high-pressure turbine and the intermediate-pressure turbine increased the distance between bearings supporting the high-pressure turbine and the intermediate-pressure turbine, becoming a major cause of the vibration of the turbine.